1. Field of the Invention
The present invention relates to equipment for testing or measuring brain activity, and may take the form of a neuropsychiatric workstation.
2. Description of the Prior Art
Please note the following discussion makes reference to publications which are detailed subsequently under the heading "Reference Publications".
Numerous studies have been undertaken into the effects of cognition on Event Related Potentials (ERPs) (see review Gevins and Cutillo 1986). The majority of these studies have utilized discrete and discontinuous stimuli such as auditory clicks, tones or the tachistoscopic presentation of visual targets. These stimuli are associated with what have been termed "transient" ERPs (Regan 1977) and constitute the familiar sequence of waveforms widely reported (e.g. McGillen & Aunon 1987) . By contrast there have been relatively few reported studies concerning cognitive effects on the evoked potentials associated with rapidly repetitive stimuli. Potentials evoked by such stimuli have been termed "Steady State Evoked Potentials" and consist of sinusoidal components at the stimulus frequency or multiples of the stimulus frequency (Regan 1977, 1989).
The Steady Stare Potential has an attractive feature relevant to the study of cognitive processes, this being the ability to assess the characteristics of the potential in as little as 10 seconds (Regan 1989) . This would make it an ideal instrument to investigate time varying phenomena in cognitive processes. In spite of this attractive feature there has been a dearth of studies demonstrating a relationship between the Steady State Evoked Potential and cognitive processes.
Galambos (Galambos & Makeig 1985, 1988) has drawn attention to the fluctuations in the human auditory Steady State Evoked Potential and has considered the relationship between these and cognitive processes. While these fluctuations were originally thought to be associated with "shifts in arousal" (Galambos & Makeig 1985), more recent reports from this group failed to uncover any relationship between cognitive processes and the auditory steady state potential (Galambos & Makeig 1988). In an extensive study, Linden et al (1987) were unable to demonstrate any effects of selective attention on the auditory steady state evoked potential. This was despite the fact that the same subjects yielded strong selective attention effects in the late components of the transient ERP.
While no feature of the auditory steady state potential has yet been demonstrated to be correlated with cognitive processes, recent studies concerning the Steady State Visually Evoked Potential (SSVEP) have yielded a relationship with cognitive processes. Wilson & O'Donnell (1986) reported that the rate of memory scanning, as determined by the Sternberg memory scanning task, (Sternberg 1969), is correlated with the apparent latency of the SSVEP. The apparent latency was calculated from the SSVEP phase versus stimulus frequency plot, a method described by Regan (1989). Specifically, subjects with shorter apparent SSVEP latencies scanned through the list of memory items faster. This occurred when the stimulus frequency was in the range 15-35 Hz. While this correlation indicated a relationship between the speed of cognitive processes and the SSVEP latency, it did not yield a relationship between a change in cognitive function, such as attention, and a corresponding change in the SSVEP. In a subsequent study by this group, the specific issue of the relationship between cognitive processes and the SSVEP was addressed when considering the effects of mental workload on the SSVEP (Wilson & O'Donnell 1988). They reported that the SSVEP appeared relatively insensitive to mental workload. This suggests a weak effect of cognitive processes on the SSVEP in the frequency ranges investigated.
Referring now to a technique known as the Probe-ERP technique, a premise is that regional increases in cortical activity associated with the cognitive processes will in turn give rise to smaller potentials evoked by an irrelevant (or probe) stimulus (Papanicolaou & Johnstone 1984). This premise is supported by findings which indicate a reduction in the transient Probe-ERP being associated with an increase in regional cerebral bloodflow (Papanicolaou 1986).
A number of Probe-ERP studies have demonstrated ERP correlates of cognitive processes. Specific examples include a finding that the attenuation of an auditory probe ERP was larger in the left hemisphere during a covert articulation task (papanicolaou et al 1983). This Probe-ERP indication of left hemisphere specialization for certain language tasks was reinforced by a more recent report indicating that auditory probe magnetic fields were more attenuated in the left hemisphere during a task involving the identification of a phonological target (Papanicolaou et al 1988). In a reading task, left temporal attenuation of the visual probe ERP was correlated with task difficulty (Johnstone et al 1984). By contrast, a visuo-spatial task requiring subjects to mentally rotate geometrical figures yielded visual probe attenuation which was greatest in the right parietal region. In another study involving a visuo-spatial task, a simultaneous measurement of regional cerebral bloodflow and the visual probe ERP demonstrated concurrent right parietal probe ERP attenuation and increased regional cerebral bloodflow (Papanicolaou et al 1987).